Body hair

Body hair, or
androgenic hair, is the
terminal hair
that develops on the human body
during and after puberty. It is differentiated from the
head hair
and less visible vellus hair, which are much finer and lighter in color. The growth of androgenic hair is related to the level of
androgens
(often referred to as male hormones; due to the production levels in males being higher on average, but it is actually present in both sexes, and therefore is a unisex hormone) in the individual.

From childhood onward, regardless of
sex, vellus hair covers almost the entire area of the
human body. Exceptions include the lips; the backs of the ears; the palms of hands; the soles of the feet; certain external
genital
areas; the navel; and
scar
tissue. The density of hair – i.e. the number of hair follicles
per unit area of skin – varies from person to person. In many cases, areas on the human body that contain vellus hair will begin to produce darker and thicker body hair, such as the first growth of beard hair
on a male and female adolescent's previously smooth chin; although it may appear thinner on the female.

Androgenic hair follows the same
growth pattern
as the hair that grows on the scalp, but with a shorter anagen phase
and longer telogen phase. While the anagen phase for the hair on one's head lasts for years, the androgenic hair growing phase lasts a few months.[1]
The telogen phase for body hair lasts close to a year.[1]
This shortened growing period and extended dormant period explains why the hair on the head tends to be much longer than other hair found on the body. Differences in length seen in comparing the hair on the back of the hand and pubic hair, for example, can be explained by varied growth cycles in those two regions. The same goes for differences in body hair length seen in different people, especially when comparing men
and women.

Like much of the hair on the human body, leg, arm, chest, and back hair begin as vellus hair. As people age, the hair in these regions will often begin to grow darker and more abundantly. This will typically happen during or after puberty. Men will often have more abundant, coarser hair on the arms and back, while women tend to have a less drastic change in the hair growth in these areas but do experience a significant change in thickness of hairs. However, some women will grow darker, longer hair in one or more of these regions.

Vellus hair grows on the chest and abdomen of both sexes at all stages of development. After puberty and extending into adulthood, most males grow increasing amounts of terminal hair over the chest and abdomen areas. Adult women also typically can grow terminal hairs around the
areola
though in many cultures these hairs are typically removed.[citation needed]

Arm hair grows on a human's
forearms, sometimes even on the elbow area. Terminal arm hair is concentrated on the wrist end of the forearm, extending over the
hand. Terminal hair growth in adolescent males is often much more intense than that in females, particularly for individuals with dark hair. In some cultures, it is common for women to remove arm hair, though this practice is less frequent than that of leg hair removal.

Terminal hair growth on
arms
is a secondary sexual characteristic in boys and appears in the last stages of puberty. Vellus arm hair is usually concentrated on the elbow end of the forearm and often ends on the lower part of the upper arm. This type of intense arm vellus hair growth sometimes occurs in young women and people of both sexes until puberty. Even though this causes the arms to appear hairy, it is not caused solely by testosterone. The hair is softer and different from men's arm hair, in texture.

Visible hair appearing on the top surfaces of the feet and
toes
generally begins with the onset of puberty. Terminal hair growth on the feet is typically more intense in adult and adolescent males than in females.

Leg hair sometimes appears at the onset of adulthood, with the legs of men more often hairier than those of women. For a variety of reasons, people may
shave their leg hair, including cultural practice or individual needs. Around the world, females generally shave their leg hair more regularly than men, to conform with the social norms of many cultures, many of which perceive smooth skin as a sign of youth, beauty, and in some cultures, hygiene. However, athletes of both sexes –
swimmers,
runners,
cyclists
and bodybuilders
in particular – may shave their androgenic hair to reduce friction, highlight muscular development or to make it easier to get into and out of skin-tight clothing.

Pubic hair
is a collection of coarse hair found in the pubic region. It will often also grow on the thighs and abdomen. Zoologist Desmond Morris
disputes theories that it developed to signal sexual maturity or protect the skin from chafing during copulation, and prefers the explanation that pubic hair acts as a scent trap. Also, both sexes having thick pubic hairs act as a sort of cushion during intercourse.[2]

The
genital
area of males and females are first inhabited by shorter, lighter vellus hairs that are next to invisible and only begin to develop into darker, thicker pubic hair at puberty. At this time, the
pituitary gland
secretes gonadotropin
hormones which trigger the production of testosterone in the testicles and ovaries, promoting pubic hair growth. The average ages pubic hair begins to grow in males and females are 12 and 11, respectively. However, in some females, pubic hair has been known to start growing as early as ages 9 and 8.

Just as individual people differ in scalp hair color, they can also differ in pubic hair color. Differences in thickness, growth rate, and length are also evident.

Islamic hygienical jurisprudence
teaches that pubic and armpit hair must be pulled out or shaven to be considered as Sunnah. Trimming is acceptable.[3]In Western culture, beginning in the 1980s, there has been a trend among women to remove or trim pubic hair, either partially or completely; the trend later spread to men as well. Fashions for both men and women to either shape or remove pubic hair rise and fall throughout much of history, though it is more often considered as a practice for women alone.[4]

Underarm hair normally starts to appear at the
beginning
of puberty, with growth usually completed by the end of the teenage years.

Today in much of the world, it is common for women to regularly
shave
their underarm hair. The prevalence of this practice varies widely, though. The practice became popular for cosmetic reasons around 1915 in the United States
and United Kingdom, when one or more magazines showed a woman in a dress with shaved underarms. As women's armpit hair contrasts more noticeably with her other body hair than does a man's, it is a significant indicator of sexual maturity and therefore unwelcome in conventional western polite society. Regular shaving became feasible with the introduction of the
safety razor
at the beginning of the 20th century. While underarm shaving was quickly adopted in some English
speaking countries, especially in the US and Canada, it did not become widespread in
Europe
until well after World War II.[5][6]
Since then the practice has spread worldwide. Some men also choose to shave their armpits.

Facial hair, as the name suggests, grows primarily on or around one's
face. Both men and women experience facial hair growth. Like pubic hair, non-vellus facial hair will begin to grow in around puberty.
Moustaches
in young men usually begin to grow in at around the age of puberty, although some men may not grow a moustache until they reach late teens or at all. For some cases facial hair development may take longer to mature than in the late teens but a lot of men experience no facial development even at an older age. Facial hair development has often been associated with stress levels.

It is common for many women to develop a few facial hairs under or around the chin, along the sides of the face (in the area of sideburns), or on the upper lip. These may appear at any age after puberty but are often seen in women after
menopause
due to decreased levels of estrogen. A darkening of the vellus hair of the upper lip in women is not considered true facial hair, though it is often referred to as a "moustache"; the appearance of these dark vellus hairs may be lessened by bleaching. A relatively small number of women are able to grow enough facial hair to have a distinct beard. In some cases, female beard growth is the result of a hormonal imbalance (usually androgen
excess), or a rare genetic disorder known as hypertrichosis.[7]
Sometimes it is caused by use of anabolic steroids. Cultural pressure leads most women to remove facial hair, as it may be viewed as a
social stigma.

Hair follicles are to varying degrees sensitive to androgen, primarily
testosterone
and its derivatives, particularly dihydrotestosterone, with different areas on the body having different sensitivity. As androgen levels increase, the rate of hair growth and the weight of the hairs increase. Genetic factors determine both individual levels of androgen and the hair follicle's sensitivity to androgen, as well as other characteristics such as hair colour, type of hair and hair retention.

Rising levels of androgen during puberty cause vellus hair to transform into terminal hair over many areas of the body. The sequence of appearance of terminal hair reflects the level of androgen sensitivity, with
pubic hair
being the first to appear due to the area's special sensitivity to androgen. The appearance of pubic hair in both sexes is usually seen as an indication of the start of a person's puberty. There is a sexual differentiation
in the amount and distribution of androgenic hair, with men tending to have more terminal hair in more areas. This includes facial hair,
chest hair,
abdominal hair,
leg hair,
arm hair, and
foot hair. Women retain more of the less visible vellus hair, although leg, arm, and foot hair can be noticeable on women. It is not unusual for women to have a few terminal hairs around their nipples as well. In the later decades of life, especially after the 5th decade, there begins a noticeable reduction in body hair especially in the legs. The reason for this is not known but it could be due to poorer circulation, lower free circulating hormone amounts or other reasons.

Androgenic hair provides tactile sensory input by transferring hair movement and vibration via the shaft to sensory nerves within the skin. Follicular nerves detect displacement of hair shafts and other nerve endings in the surrounding skin detect vibration and distortions of the skin around the follicles. Androgenic hair extends the sense of touch beyond the surface of the skin into the air and space surrounding it, detecting air movements as well as hair displacement from contact by insects or objects.[8][9]

Determining the evolutionary function of androgenic hair must take into account both
human evolution
and the thermal properties of hair itself.

The thermodynamic properties of hair are based on the properties of the
keratin
strands and amino acids
that combine into a 'coiled' structure. This structure lends to many of the properties of hair, such as its ability to stretch and return to its original length. This coiled structure does not predispose curly or frizzy hair, both of which are defined by oval or triangular hair follicle cross-sections.[10]

Hair is a very good thermal conductor and aids heat transfer both into and out of the body. When
goose bumps
are observed, small muscles (Arrector pili muscle) contract to raise the hairs both to provide insulation, by reducing cooling by air convection of the skin, as well as in response to central nervous stimulus, similar to the feeling of 'hairs standing up on the back of your neck'. This phenomenon also occurs when static charge is built up and stored in the hair. Keratin however can easily be damaged by excessive heat and dryness, suggesting that extreme sun exposure, perhaps due to a lack of clothing, would result in perpetual hair destruction, eventually resulting in the genes being bred out in favor of high skin
pigmentation. It is also true that parasites can live on and in hair thus peoples who preserved their body hair would have required greater general hygiene to prevent diseases.[11]

Markus J. Rantala of the Department of Biological and Environmental Science,
University of Jyväskylä,
Finland, said humans evolved by "natural selection" to be hairless when the trade off of "having fewer parasites" became more important than having a "warming, furry coat".[12]

P.E. Wheeler of the Department of Biology at
Liverpool Polytechnic
said quadrupedalsavannahmammals
of similar volume to humans have body hair to keep warm while only larger quadrupedal savannah mammals lack body hair, because their body volume itself is enough to keep them warm.[13]
Therefore, Wheeler said humans who should have body hair based on predictions of body volume alone for savannah mammals evolved no body hair after evolving bipedalism
which he said reduced the amount of body area exposed to the sun by 40%, reducing the solar warming effect on the human body.[13]

Decreased body hair began in hominins over 2 million years ago and aided
persistence hunting
(the ability to catch prey in very long distance chases) in the warm savannas where hominins first evolved. The two main advantages are felt to be bipedal locomotion and greater thermal load dissipation capacity due to better sweating and less hair.

Cambridge University zoologist Charles Goodhart believed men have long preferred the "hairless trait" in women, ever since the existence of the "hairless trait" occurred in our hairy forebears 70,000–120,000 years ago during the last episode of global warming. Goodhart argued that humans are relatively hairless today compared to our ancestors because women who were sexually selected for their "hairless trait" passed it on to both their male and female offspring.[14]

In 1876,
Oscar Peschel
said that North Asiatic Mongols, Native Americans, Malays, Hottentots and Bushmen have little to no body hair. Peschel said that Semitics and Indo-Europeans have a lot of body hair, and Peschel said that Southern Europeans have a lot of body hair, especially Portuguese and Spaniards.[16]

Rodney P.R. Dawber of the Oxford Hair Foundation and Clinical Lecturer in Dermatology said East Asian males have little or no facial or body hair and Dawber also said that
Mediterranean
males are covered with an exuberant pelage.[18]

C.H. Danforth and
Mildred Trotter
of the Department of Anatomy at Washington University
did a study using army soldiers of European origin where they concluded that dark-haired white
men are generally more hairy than fair-haired white men.[20]

Milkica Nešić et al. of the Department of Physiology at the
University of Niš,
Serbia, cited prior studies to indicate that the frequency of hair on the middle finger joint (mid-phalangeal
hair) in whites
is significantly higher than in black
populations.[22]

Eike-Meinrad Winkler and Kerrin Christiansen of the Institut für Humanbiologie,
Hamburg,
Germany, did a study using
Kavango
people and !Kung
people of body hair and hormone levels to investigate the reason black Africans
did not have bodies as hairy as Europeans.[23]
Winkler and Christiansen concluded the difference in hairiness between black Africans and Europeans had to do with differences in androgen
or estradiol
production, in androgen metabolism, and in
sex hormone
action in the target cells.[23]

Valerie Anne Randall of the Department of Biomedical Sciences,
University of Bradford, said beard growth in Caucasian men increases until the mid-thirties due to a delay caused by growth cycles changing from
vellus hair
to terminal hair.[24]
Randall said white men and women are hairier than Japanese men and women even with the same total plasma androgen
levels.[24]
Randall says that the reason for some people being hairy and some people not being hairy is unclear, but that it probably is related to differing sensitivity of hair follicles
to 5α-reductase.[24]

It has shown that individuals can be uniquely identified by their androgenic hair patterns, e.g. even when face and tattoos, etc., are covered, persons can still be identified by hair on other parts of the body.[27][28]